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IRON AND STEEL STRUCTURES, 



ZEFIFIEOT OIF 

MAGNETIC AND GALVANIC FORCES 



C. M. CRESSON, M. D 



JULY, 187i 



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THE EFFECT OF MAGNETIC AND GALVANIC FORCES UPON 

THE STRENGTH OF, AND DESTRUCTION OF IRON AND 

STEEL STRUCTURES. 

By Chables M. Ceesson, M.D. 

{Read before the Americaii Philosophical Society, June 18, 1875.) 

Bars and Structures of Iron and Steel when allowed to remain at rest 
for a considerable time acquire measurable magnetic polarity. 

Moderate percussion, alternations of heat and cold, exposure to the 
rays of the sun, especially with a long axis of figure parallel, or nearly 
coincident with a magnetic meridian of the earth have a tendency to de- 
velop and strengthen magnetic polarity. 

Thus, Iron Bridges, Iron Vessels upon the stocks in progress of con- 
struction, and Iron Railway Tracks are particularly liable to acquire 
magnetic polarity. 

It is asserted that the relative position of the long axis of Iron Ships 
with reference to the magnetic meridian materially affects their polarity 
and the facility of the correction of their compasses. 

If the keels of such vessels be laid on a North and South line, they are 
supposed to acquire greater polarity, and to retain it more steadily than 
when laid East and West. 

The evidence of an iron ship's polarity is exhibited to the greatest de- 
gree, by comparison of its effects upon its compasses when the vessel is 
sailing in an easterly or westerly direction. 

A consideration of the following facts seems to favor the conclusion 
that magnetic bars of Iron should be better able to resist tensile strain 
than those which are not magnetic. 

A thoroughly magnetic bar is one of which each end repels a pole of a 
magnetic needle. The centre of such a bar is neutral, that is attracts 
either end of a magnetic needle and repels neither. 

If we break such a bar in half, we are possessed of two magnetic bars ; 
that end of the original bar which attracted the south end of a magnetic 
needle continues to attract it, that which attracted the north end continues 
to do so, whilst the two new ends which had formed the ii^tral centre of 
the original bar, each acquires a polarity opposite to mmk other, and also 
opposite to that possessed by its own opposite end. A continuance of this 
process, that is, the fracturing of each half until we have obtained such 
minute fragments of the bar as can be examined only under the micro- 
scope, still produces perfectly polarized bars, possessing all of the mag- 
netic characteristics of the original bar, with varying, attracting, and 
repelling force according to some ratio of the relative length and thick- 
ness of the fragments. 

Arguing upon this we are led to the conclusion that a continuance of 
this process must produce molecular magnets. 

If we place magnetic bars in contact with each other, the north and 
south poles alternating and in contact with each other we obtain a metallic 



2 

chain of considerable strength, although its corai)onent parts are not 
mechanically connected together. The closer the contact of the ends of 
the bars the stronger will be the chain. 

If with isolated bars we can obtain a connecting force equal to many 
pounds by close contact, how much stronger must be the connecting force 
when exerted between molecule and molecule. 

Such an argument undoubtedly leads to the conclusion that bars satu- 
rated with magnetic force should certainly be stronger than those that 
are not. 

Faraday announced that ""there existed lines of force within the mag- 
net of the same nature as those without. What is more they are exactly 
equal in amount with those without. They have a relation in direction 
to those without ; in fact are continuations of them, absolutely unchanged 
iu their nature." 

To determine the effect of magnetic force upon the tensile strength of 
Iroa and Steel,* bars of each were selected and cut into suitable lengths 
for use in the breaking machine and numbered. 

Nos. 1, 3, 5, &c., were broken in the usual manner. 

Nos. 2, 4, 6, &c., w^hilst in the breaking machine were surrounded by 
a suitable coil of copper- wire, through which a current of galvanic elec- 
tricity was passed during the operation of breaking. 

The results obtained from the magnetic Steel bars were about one per 
cent. less than those obtained from the non-magnetic, and from the mag- 
netic soft Iron bars about three per cent, less than from the non-magnetic. 

Both the Steel and Iron bars became heated whilst within the influence 
of the current of electricity, the soft Iron more so than the Steel. 

It occurred to me that the depreciation of strength might have been 
caused by the rise of temperaturef in the bars, and I accordingly prepared 
permanent magnets from alternate sections of a steel bar and repeated 
the experiments comparing the cold magnets with the unmagnetized sec- 
tions of the same bar. The results showed no appreciable difference in 
strength between the magnetic and non-magnetic sections. 

To test the matter still further, bars of Steel were so magnetized as to 
present a pole at one end, the other in the middle of the bar, with one 
end neutral, that is, one end of the bar attracted the North or South pole 
of a magnetic needle and repelled the South or North, and the other end 
of the bar attracted either pole of a magnetic needle. 

* The Steel employed in the experiment was " Jessop's Round Machinery," >4 iJ^c^ 
rod— 

and broke at \ maximum, 127,934 lbs. 
and Dioke at j minimum, 126,694 lbs. 

per square inch of section. 

rri>« fr.^^ u^n.\rr. „» S niaxlmum, 59,948 lbs. 
The Iron broke afe J ^^■,,,^^^,^; 66:887 lbs. 

per square inch of section, 
t For effects of temperature upon the tensile strength of Iron, see Report of the Com- 
mittee MiiKiMaiVOT^MfllPWMfeMv of the Franklin Institute of Pennsylvania,— " upon the 
strength of materials employed in the construction of Steam Boilers." Kxporimeuts 
made at the rocjuest of the Treasury Department of the Ignited States (Jan'y 4th, 1831— 
Jan'y 5th, 1837). 



Under these conditions if there was any effect to be had from the in- 
fluence of the magnetic force, the bar should incline to break either at 
the central pale or at the neutral point between the poles. 

The result's of the experiments showed that there was no inclination to 
a choice of either location as the place of fracture. 

The conclusion arrived at, is, that the condition o-f magnetic polarity 
does not in any way influence the strength of steel bars. With refer- 
ence to the soft iron bars the comparison was not made, for the reason 
that they would not remain magnetic unless surrounded by the galvanic 
coil, in which case they became heated by the action of the current. 

How far a change from fibrous to crystalline structure is effected by the 
influence of magnetism has not been ascertained, or whether there is any 
deterioration of the strength of iron or steel on such account. 

Iron telegraph wires, in the course of time become brittle, and to such 
an extent that if the usual method of uniting them by winding each upon 
the other is attempted, they are frequently broken in the process. 

From this it would appear that the passage of a strong galvanic cur- 
rent produces some molecular change affecting the strength of iron. 
Such conducting wii-es, however, are not necessarily or even usually mag- 
netic. There can be no doubt, however, as to the deteriorating effect of 
galvanic force as an accelerator of oxidation or the solution of a metal. 

Observations upon Iron Bridges and structures subjected to atmos- 
pheric influences and upon Boilers exposed to the action of heat and the 
chemical agents contained in ordinary waters lead to the conclusion that 
galvanic force is usually as great, and frequently a far greater cause of 
deterioration ^than mechanical wear. Indeeed all of the operations of 
nature, organic and inorganic, both constructive and disjunctive, involve 
the production of more or less galvanic force or are the results of its 
action. 

Motion, unaccompanied by any other apparent change than that of 
place, is a disturber of electric or galvanic equilibrium, and the converse 
is equally true. If it were possible to produce perfectly pure and homo- 
geneous iron, then the generation of destructive galvanic currents by the 
contact of sheets or bars would not take place. 

By exercising care in the selection of iron, especially that used for 
steam boilers, the deterioration from galvanic action can be reduced to a 
minimum. 

Many steam boilers have come under my observation in which the cor- 
rosion was but slight, and affected all parts equally, others in which the 
metal of a single sheet only was attacked, the corrosion of which sheet 
protected the remainder of the boiler almost as eJ0S.ciently as if the sheet 
had been replaced by one of the metal zinc. 

The most striking instance of the effect of introducing a sheet of metal 
of greatly differing electro-condition, that occurs to me, is that of a 
boiler which had been in use for a considerable length of time without 
showing any unusual tendency to corrosion, when from some cause it be- 
came necessary to replace a sheet by a new one. 



The result of the introduction of a new sheet was to set up at once a 
strong galvanic action by which every sheet in the boiler was corroded 
except the new one. 

Samples of iron cut from the edges of the old and from the new sheets 
were placed in a bath to which a few drops of dilute acid were added and 
a connection made with a galvanometer, resulting in the production of a 
strong current ; the purer iron corroding, and protecting that which con- 
tained the greatest amount of carbon. 

The inciting cause of the galvanic action was therefore judged to be 
the introduction of a sheet of iron electro-negative to those already in 
the boiler, its position in the electro-chemical scale depending upon the 
amount of carbon it contained. 

The injurious effect consequent upon the j auction of masses of wrought 
iron of varying electro-chemical properties, is, therefore, increased when 
steel is joined to wrought iron, as is frequently the case in locomotive 
boilers in the tubes and tube sheets. 

Again by the junction of cast iron to steel or to wrought iron, the de- 
structive effect is greatly intensified, and at times becomes quite as vio- 
lent as when copper is made an element in the galvanic circuit in con- 
nection with wrought iron. 

The necessity for the selection of iron with reference to its electric con- 
dition, applies equally to the material employed for Bridges or Vessels or 
Boilers or any structure which is to be built up from separate sheets and 
bars of iron. 

It is or ought to be the habit of careful constructors to cut sample 
pieces from every sheet or bar of metal worked, and to make a trial of 
their quality by bending hot and cold, and to make frequent tests of ten- 
sile strength. Examinations as to electro-chemical condition can be 
made with equal facility. Determinations of the composition of the 
metal or of the percentage of carbon in it by chemical analysis are un- 
necessary ; an ordinary workman furnished with a coarse galvanometer 
and a weak acid bath can ascertain the exact electro-condition of each 
sheet or bar more rapidly than he can examine the quality by the ordinary 
tests of bending on an anvil, hot and cold. With the metal of Bridges, 
Vessels, and especially Steam Boilers, the deterioration by corrosion is 
more to be feared than is mechanical wear. 

Galvanic corrosion acts with greater vigor in locations that are usually 
inaccessible, such as the interior of joints or defective sheets or parts that 
are closely approximated, and the mischief is only suspected when it has 
progressed to such a degree as to become evidently dangerous and the 
parts are in condition to require immediate attention and repair. 

Attention to the precautions enumerated for securing mechanical and 
chemical fitness of the metal to be used for structures of iron, will un- 
doubtedly promote economy and safety. 



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